The present invention relates to a process for preparing aldehydes in the presence of a catalyst based on rhodium and in the absence of complex-forming organophosphorus compounds in a reaction zone, wherein a rhodium-containing organic solution which has been mixed beforehand with at least one organic acid or mixture thereof is fed to the reaction zone.
Hydroformylation or the oxo process is the transition metal-catalysed reaction of olefins or olefinically unsaturated compounds with hydrogen and carbon monoxide to form aldehydes and alcohols which have one more carbon atom than the olefin used. The hydroformylation process has now attained considerable economic and industrial importance. The aldehydes obtained initially in this process are used as such or represent valuable intermediates for the production of, for example, alcohols, carboxylic acids, esters or amines.
Hydroformylation is catalysed by hydridometal carbonyls, preferably those of metals of transition group VIII of the Periodic Table of the Elements. Apart from cobalt, the classic catalyst metal, catalysts based on rhodium have been increasingly used for some years. In contrast to cobalt, rhodium allows the reaction to be carried out at a relatively low pressure. In addition, when terminal olefins are used, straight-chain n-aldehydes are preferentially formed and isoaldehydes are formed to only a minor extent. Finally, the hydrogenation of the feed olefins to saturated hydrocarbons is also significantly lower in the presence of rhodium catalysts than when using cobalt catalysts.
Hydroformylation of olefinically unsaturated compounds is carried out industrially in the presence of rhodium carbonyl complexes with tertiary organic phosphine or phosphite ligands as catalysts. In one process variant, the reaction is carried out in a homogeneous phase, i.e. feed olefin, catalyst and reaction products are present together in solution. The reaction products are usually separated off from the mixture by distillation, more rarely by other processes such as extraction. The hydroformylation process carried out in the homogeneous phase can be in the form of a gas recycle process as described in U.S. Pat. No. 4,247,486 or in the form of a liquid recycle process as described in U.S. Pat. No. 4,148,830.
In a further process variant, the rhodium-catalysed hydroformylation reaction can also be carried out in the absence of complex-forming ligands, for example phosphines or phosphites. Such rhodium catalysts which have not been modified with phosphines or phosphites and their suitability for hydroformylation catalysts are known from the literature and they are referred to as unmodified rhodium catalysts. It is assumed in the technical literature that the rhodium compound HRh(CO)4 is the catalytically active rhodium species in hydroformylation using unmodified rhodium catalysts, although this has not been conclusively proven because of the many mechanisms proceeding simultaneously in the reaction zone. The unmodified rhodium catalysts are formed from rhodium compounds, for example rhodium salts such as rhodium(III) chloride, rhodium(III) nitrate, rhodium(III) acetate, rhodium(II) acetate, rhodium(III) sulphate or rhodium(III) ammonium chloride, from rhodium chalcogenides such as rhodium(III) oxide or rhodium(III) sulphide, from salts of rhodium oxo acids, for example rhodates, from rhodium carbonyl compounds such as Rh4(CO)12 and Rh6(CO)16 or from organorhodium compounds such as rhodium carbonyl acetonylacetonate, cyclooctadiene rhodium acetate or chloride in the presence of carbon monoxide/hydrogen mixtures, also referred to as synthesis gas, in the reaction zone under the conditions of the hydroformylation reaction. Here, the rhodium compound can be used as solid or advantageously in solution. Hydroformylation processes carried out in the presence of unmodified rhodium complexes are known, for example, from DE 38 22 038 A1, in which rhodium 2-ethylhexanoate is used, or from EP 0 695 734 A1, according to which a solution of a previously formed rhodium carbonyl compound in the olefinically unsaturated compound to be reacted is used.
Owing to the absence of stabilizing ligands, precipitation of metallic rhodium from the crude hydroformylation mixture obtained by the unmodified process variant and taken from the reaction zone can occur during the work-up of the mixture by distillation. The rhodium precipitated in the work-up apparatus for the crude hydroformylation mixture cannot be recirculated to the hydroformylation process and therefore leads to rhodium losses which, owing to the high prices of noble metal, represent an economic disadvantage. To reduce rhodium losses in the work-up stage of a crude hydroformylation mixture which has been obtained by unmodified rhodium-catalysed hydroformylation, EP 0 695 734 A1 proposes firstly carrying out an extraction with an aqueous solution of a water-soluble phosphorus-containing complexing agent, with rhodium being extracted into the aqueous phase and aldehyde or alcohol being isolated from the remaining hydroformylation mixture. The aqueous extract is subsequently treated with an organic liquid in the presence of carbon monoxide or gases containing carbon monoxide under superatmospheric pressure at elevated temperature, resulting in rhodium going as rhodium carbonyl into the organic phase which can subsequently be recirculated to the reaction zone.
Apart from the rhodium precipitates which can occur in the work-up of a crude hydroformylation mixture obtained by unmodified rhodium catalysis, precipitation of rhodium metal close to the inlet region can also be expected when the rhodium-containing solution is introduced into the reaction zone. This precipitated rhodium metal is no longer converted into catalytically active rhodium carbonyl in the reaction zone, even under synthesis gas pressure. As a result, there is firstly only a smaller amount, based on the rhodium used, of catalytically active rhodium available and, secondly, the precipitated rhodium metal remains in the reaction zone and leads to rhodium losses. According to DE 19 20 960 A1, heating of the rhodium solution and of the mixture of olefinically unsaturated compound and rhodium solution in the absence of carbon monoxide has to be avoided in a continuously operated, unmodified hydroformylation process. Likewise, no high rhodium concentrations should occur during mixing of the rhodium solution with the olefinically unsaturated compound in the reaction zone. DE 19 20 960 A1 therefore recommends intimately mixing synthesis gas, the olefinically unsaturated compound and the rhodium solution in the vicinity of the inlet into the reaction zone. Here, a solution of rhodium compound, for example rhodium chloride or nitrate, in a polar organic solvent is fed to the reaction zone, with the rhodium compound preferably being insoluble in the olefinically unsaturated compound. For example, use is made of a solution of rhodium acetate in a mixture of methanol and acetic acid, in acetic acid or in propionic acid. The known continuously operated hydroformylation process allows conversions of 1-octene of 94-96% at a residence time of about 2 hours and a rhodium concentration of 14 ppm, with rhodium precipitates in the vicinity of the inlet for the rhodium solution into the reaction zone being able to be reduced at the same time. It is likewise pointed out that the rhodium compound added should be insoluble or virtually insoluble in the olefinically unsaturated compound in order to avoid high rhodium concentrations during mixing of the rhodium solution with the olefinically unsaturated compound. If locally high rhodium concentrations occur, precipitation of rhodium metal can occur in the presence of the olefinically unsaturated compound.